refactor(layout): extract AppNav and UserMenu sub-components

Split +layout.svelte (205 lines) into:
- AppNav.svelte: logo + nav links with active-state styling
- UserMenu.svelte: avatar button, dropdown, click-outside handler

Layout drops from 205 → 80 lines.

Part of #75

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/UserMenu.svelte

@@ -0,0 +1,81 @@
+<script lang="ts">
+import { enhance } from '$app/forms';
+import { m } from '$lib/paraglide/messages.js';
+
+let { userInitials }: { userInitials: string | null } = $props();
+
+let userMenuOpen = $state(false);
+
+function clickOutside(node: HTMLElement) {
+	const handleClick = (event: MouseEvent) => {
+		if (node && !node.contains(event.target as Node) && !event.defaultPrevented) {
+			userMenuOpen = false;
+		}
+	};
+	document.addEventListener('click', handleClick, true);
+	return () => {
+		document.removeEventListener('click', handleClick, true);
+	};
+}
+</script>
+
+<div
+	class="relative"
+	{@attach clickOutside}
+	onkeydown={(e) => {
+		if (e.key === 'Escape') userMenuOpen = false;
+	}}
+	role="none"
+>
+	{#if userInitials}
+		<button
+			type="button"
+			aria-expanded={userMenuOpen}
+			aria-haspopup="true"
+			onclick={() => (userMenuOpen = !userMenuOpen)}
+			class="flex h-8 w-8 items-center justify-center rounded-full bg-primary font-sans text-xs font-bold text-white transition-opacity hover:opacity-80"
+		>
+			{userInitials}
+		</button>
+	{:else}
+		<button
+			type="button"
+			aria-label={m.nav_profile()}
+			aria-expanded={userMenuOpen}
+			aria-haspopup="true"
+			onclick={() => (userMenuOpen = !userMenuOpen)}
+			class="inline-flex items-center gap-1.5 px-3 py-2 font-sans text-xs font-bold tracking-widest text-ink-3 uppercase transition-colors hover:text-ink"
+		>
+			<img
+				src="/degruyter-icons/Simple/Small-16px/SVG/Action/Account-SM.svg"
+				alt=""
+				aria-hidden="true"
+				class="h-4 w-4 opacity-50"
+			/>
+		</button>
+	{/if}
+
+	{#if userMenuOpen}
+		<div
+			class="absolute top-full right-0 z-50 mt-1 min-w-[10rem] rounded-sm border border-line bg-overlay shadow-md"
+		>
+			<a
+				href="/profile"
+				onclick={() => (userMenuOpen = false)}
+				class="block px-4 py-2.5 font-sans text-xs font-bold tracking-widest text-ink-2 uppercase transition-colors hover:bg-muted hover:text-ink"
+			>
+				{m.nav_profile()}
+			</a>
+			<div class="border-t border-line">
+				<form action="/logout" method="POST" use:enhance>
+					<button
+						type="submit"
+						class="w-full px-4 py-2.5 text-left font-sans text-xs font-bold tracking-widest text-ink-3 uppercase transition-colors hover:bg-muted hover:text-ink"
+					>
+						{m.nav_logout()}
+					</button>
+				</form>
+			</div>
+		</div>
+	{/if}
+</div>